Profile

Construction Corner: Ultrasounds assess health of concrete structures

Mention ultrasound to many people and they’ll think of medical scans ordered by their family doctor.

Engineers, on the other hand, might think of non-destructive testing of concrete structures.

That’s an application that has grown in use over the last 15 years. The available devices, which examine the internal condition of concrete structures, use any of several tweaks of the basic technology.

Now, researchers at the Oak Ridge National Laboratory in eastern Tennessee are using something called ultrasonic linear arrays as they search for ways to measure the long-term structural integrity of American nuclear power plants. In other words, they’re figuring out ways to give those plants an ultrasound scan.

What they hope to do is come up with a way to simulate the aging process that creates alkali-silica reaction in concrete. The reaction, usually referred to simply as ASR, is a slow chemical reaction that creates pockets of gel within concrete structures. As these pockets expand, they cause small cracks.

What triggered the research project was the age of the country’s nuclear plants.

When the plants were built they were licensed for 40 years of operation. Most of them now are approaching that age or are already past it. But the country’s Nuclear Regulatory Commission began to think about the safety implications of extending the licenses to 60 years. The commission found that a 20-year extension wouldn’t be a problem. But can they go from 60 years to 80? 100? At that age the science isn’t as clear.

Right now scientists think that at about the 80-year mark there might be visible signs of degradation in the plants’ concrete containment structures. But to be able to discover degradation in any specific structure, they will need an instrument that can check that structure’s internal health.

There are devices available that can probe things like the concrete members in buildings and bridge decking, but those things are typically about 30 centimetres thick. The containment walls in a nuclear plant are more like a metre thick — and heavily reinforced.

Dwight Clayton is the engineer leading the research team. In an article appearing in the Oak Ridge internal magazine, he said that the "depth of the concrete and the amount of reinforcement, make it hard to ‘see’ though with any type of technology.

"The sound waves produced by (the ultrasonic linear arrays) go into the concrete, are reflected off whatever is in the wall, and return. So they have to travel through as much as two metres of concrete."

In, then out.

The first thing Clayton and his team had to do was develop robust signal-processing software to pull data signals out of the "noise" created by the reinforcing material. That allows the team to see defects in the concrete more clearly.

Then, with the help of a team from the University of Tennessee, a wall system was developed to simulate a nuclear plant wall. The mock-up contains various defects so researchers can find out how the system reacts to different problems, like delamination. The wall was also deliberately designed to develop ASR within months, not years. That will let the team better understand the mechanisms of ASR and fine-tune their ultrasonic equipment.

"By applying elevated temperature and 100-per-cent humidity to the test wall, we’ll be able to accelerate the process," Clayton said. "Two years of testing will simulate a much longer time in the life of a normal concrete wall."

The hope, he said, is to be able to roll out a concrete structure evaluation system that can be deployed by 2020. And it won’t be limited to nuclear power plants. It could also be applied to all sorts of structures with thick concrete masses, including bridges, stadiums and dams.

It seems we still have much to learn about how the world’s most commonly used building material behaves.